The present invention relates to a thermoplastic polyester resin composition. More in detail, the present invention relates to a thermoplastic polyester resin composition excellent in ultraviolet screening, comprising a thermoplastic polyester resin and an amount effective in ultraviolet screening of at least one of naphthalenetetracarboxylic acids, acid anhydrides thereof, imides thereof and esters thereof.
According to the excellent mechanical properties and chemical specificities of polyester represented by polyethylene terephthalate, the polyester has been broadly used for fibers, films, etc. Moreover, according to the excellent transparency, the gas-barrier property, the safety and sanitation properties of the polyester, the suitablity of the polyester as the container of carbonated-beverage drinks, fruit juice drinks, liquid seasonings, edible oils, liquors and wines has attracted an attention in recent years. Still more, from the viewpoint of energy-saving and a countermeasure for earthquake, as the new use of the polyester film, the use of the polyester as a film sticked to window for the heat-radiation screening or the prevention of glass scattering has shown the rapid development. Besides, in the field of agriculture and horticulture, the demand for providing a transparent film selectively screening ultraviolet light (UV light) of a specified wavelength range is strong with the object of growth-promotion and increase of harvest of the crops for instance, refer to Japanese Patent Application Laid-Open (KOKAI) No. 53-98242 (1978).
Although these containers and films of polyester are excellent in screening UV light up to 320 nm wavelength range, they are almost transparent to UV light or they almost transmit UV light above the longer wavelength range and visible radiation. For instance, in the case where an edible oil or a liquid seasoning such as sweet sake, dressing, etc. is filled in a container made of polyester and the container is preserved for a few months, there are many cases where the deterioration of the content of the container, for instance, a delicate change of the colour, taste and flavour, occurs gradually according to the preserving conditions although there are specific cases according to the respective foodstuff filled in the container and to the preserving conditions.
Although the deterioration of the content of the container originates in the external causes such as oxygen, heat, light, particularly ultraviolet, microorganisms, etc., in the case of the polyester container, since it is relatively excellent in oxygen gas-barrier property if it is possible to further improve the ultraviolet screening property of the polyester container, it becomes possible to prevent remarkably the deterioration of the content even for a long term preservation.
Furthermore, in the case of using the polyester film for sticking to window, for instance, when the UV light in the sun light is transmitted, the discoloration of the fixtures within the house such as furnitures, books, household implements, etc. is caused and accordingly, it is necessary to prevent the transmittance to the minimum or to completely screen the UV light. In the case of mulching cultivation in agriculture and horticulture, it has been known to be possible that the growth of many useful plants is promoted, the crops of high quality can be harvested in an early season and in a large amount by mulching with the transparent coating material which screens substantially the transmittance of the UV light of a wavelength of not longer than 370 nm for instance, refer to Japanese Patent Application Laid-Open (KOKAI) No. 53-124556 (1978).
At present, in the fields, an ultraviolet absorbing agent is generally used and added to the polyester for this purpose. However, since the ultraviolet absorbing agent is generally expensive and the adding step is complicated, and moreover, such an ultraviolet absorbing agent is large in sublimation generally and there are many agents of poor in heat-stability, there are cases of causing troubles in the adding step and at the time of mold-processing, and there is a fear of moving the agent to the content of the container in the case where such an agent is used in the foodstuff container and the packaging of foodstuff. Namely, the use of such an ultraviolet absorbing agent is not necessarily favorable.
As a result of the present inventors' earnest studies on the polyester resin composition which meets with the above-mentioned object, it has been found that a polyester resin composition comprising a thermoplastic polyester resin and an amount effective in ultraviolet screening of at least one of naphthalenetetracarboxylic acids, acid anhydrides thereof, imides thereof and esters thereof, can sufficiently screen the UV light in the longer wavelength range as well as the UV light in the shorter wavelength range, and on the basis of the finding, the present invention have been attained.